ABSTRACT
Mammalian brains are protected by a four-tier system comprised of (1) an outer protective layer derived from the skull, (2) protection from the skull by the envelope membrane system, meninges (the dura, arachnoid, and the pia mater), (3) a structure of connective tissue separating the skull from the brain, which contains the arachnoid, a subarachnoid space that contains the protective buffering cerebrospinal uid, and (4) the blood-brain barrier that protects the brain from blood-based toxins. At the cellular level, the brain is composed of neurons that transfer and retain information, and the identication of novel neurons such as mirror neurons suggests that the brain is by no means fully characterized (1). The “work horses” are the multifunctional glia cells, which are present at a ratio of 10:1 (glia/neurons) (2). The glia cells provide a support network to neurons, while the multifunctional astrocytes (a subtype of glia cells) control neuronal signals using chemical mediators (3). Microglia cells are the “macrophages of the brain” and arise from myeloid progenitor cells. A host of neurotransmitters regulate signaling by glia cells, such as noradrenaline (norepinephrine), acetylcholine, dopamine, and glutamate (4).
